Estimates by the World Health Organization indicates that 80 % of the global population use herbal medicines for prophylaxis and curative purposes. In rural Kenya, Traditional Medicine is the primary source of healthcare, and most often the only source of healthcare service. This is due to its ease of accessibility, affordability and trust by millions of people. The cost factor of most phytomedicines makes them all the more agreeable at a time of spiralling healthcare expenses and nearly widespread austerity. Even though conventional medicine exists concomitantly with Traditional Medicine, phytomedicines have often been popular for cultural and historical reasons. Herbal medicinal products have turned out to be commercially widely available, particularly in industrialised nations. The rationale for the use of Ocimum americanum L. (Lamiaceae) in antimicrobial phytotherapy is largely based on the long-term experience of traditional medicine practitioners. This study aimed to investigate the antimicrobial activity and cytotoxicity of crude extracts and their fractions in a microbial and brine shrimp model. Several solvents were selected, their crude extracts and fractions evaluated for their antimicrobial activity, cytotoxicity and phytochemical composition. These included: aqueous, acetone, 70 % hydroethanolic, chloroform and ethyl acetate. Standard bacterial strains of Bacillus cereus (ATCC 11778), Staphylococcus aureus (ATCC 25925), Escherichia coli (ATCC 25922), Klebsiella pneumoniae (ATCC 700603) and one fungal strain, Candida albicans were used to assess the Minimum Inhibitory Concentration and Minimum Bactericidal Concentration of the Ocimum americanum L. sample extracts and their fractions via standard antimicrobial procedures at the microbiology laboratory, Department of Public Health, Pharmacology and Toxicology University of Nairobi. The data was evaluated for susceptibility of bacterial species and considered significant at 95 % confidence interval. Cytotoxicity of the crude samples and fractions were analysed using brine shrimp lethality test with ten-fold dilutions of 1000 µg/mL, 100 µg/mL and l0 µg/mL. Median Lethal Concentration at p < 0.05 confidence intervals was determined using Probit analysis. Established xiiphytochemical screening tests were performed to show the presence or absence of secondary metabolites. Cardiac glycosides, flavonoids, tannins, phenolics and reducing sugars were present in all sample extracts and fractions while polyuronides were absent. Two crude extracts and their fractions exhibited activity against the tested microorganisms. The hydroethanolic extract and its fractions were most active against the tested microbes. There was no significance difference (p > 0.05) in antibacterial activity between the acetonic and hydroethanolic extracts and their fractions. Gram-positive bacteria were more susceptible to the extracts/fractions than Gram-negative microbes. Bacillus cereus was most susceptible while Escherichia coli exhibited the highest resistance. All the sample extracts had statistically significant (p < 0.05) cytotoxicity at LC50 < 1000 µg/mL. Amongst all the samples the fractions of aqueous alcohol crude samples were more prospective thus good candidate for further research. Chloroform fraction of hydroethanolic sample extract was highly toxic with LC50 value of 0.59 µg/mL. The ethyl acetate sample fractions of aqueous alcohol crude sample have demonstrated promising antimicrobic effects against the Gram-positive microorganism Bacillus cereus. The fractions of hydroethanolic crude samples have potential bioactive molecules which are accountable for antimicrobial and cytotoxicity properties respectively. Results from the present research will provide a groundwork for finding an innovative natural phytomedicine. However extensive study is needed to quantify, isolate and characterise the phytocompounds.

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